Digital imaging systems have improved the process of creating, editing and rendering images. In particular, such systems have decreased the amount of processing time necessary to produce a final product. Nonetheless, the ability of digital imaging systems to process and, particularly, to render images is limited by the memory capacity of the systems.
In digital imaging systems, an image is often divided into fixed spatial coordinates on a rectangular grid where each sample point has one color and is referred to as a picture element, commonly known as a pixel or a “dot” (not to be confused with the halftone dot used in the printing industry). Such an image is usually referred to as a raster image and is typically represented and stored in a format which uses one or more bits per pixel to identify the color of each pixel. The total amount of data necessary to represent an image depends on several factors, some of which include the image size, the resolution of the image, and the number of bits per pixel.
Large high-resolution images, particularly those containing “continuous tone” or “contone” content (i.e., 8 or more bits per color component per pixel), require an extensive amount of data to represent the images. Because imaging rendering devices have a limited memory and processing capacity, such large high-resolution images often place a demand on the image rendering devices that exceeds their image rendering capabilities. As an example, a Raster Image Processor (RIP) would typically not be able to handle the volume of data in a printing format of a 1200 dot per inch (dpi) image file represented in contone raster format, such as what might be required for imaging data for a map. For example, a standard printing form sized 32 inches by 44 inches full of 1200 dpi, 8 bits-per-pixel images would require about 2 gigabytes of memory, well beyond that available on most rendering/RIP workstations. However, in many practical applications, the image consists of some photographic content and a large portion of “line work”, i.e., text or geometric objects which delineate areas of constant color which are amenable to representation with a small number of bits per pixel.
Therefore, a need exists for a method of reducing the volume of data representing such high resolution images so that the images can be adequately processed and rendered in digital imaging systems.